Power sources that tap into the energy released in radioactive decay offer the potential for long-lived lightweight power sources that are independent of environmental energy sources such as solar or wind power, and that do not require refueling or recharging. However, such radiation manifests in the form of high energy neutral or charged particles (photons, neutrons, electrons, positrons, or alpha particles), gamma rays, and X-rays which are not suitable for directly powering sensors, electrical circuits, or other manmade devices. What is needed is an efficient, reliable, compact and lightweight technology for converting the energy contained in the radioactive emissions into usable electrical power. Many methods have been attempted, but so far all have significant shortcomings.
At the megawatt and larger scale, which extends for example from nuclear powered submarines through commercial nuclear power plants, thermo-mechanical conversion is the preferred approach. However, this technology does not scale well to small portable (<100 kg) generators. Furthermore, the complex mechanical components require regular maintenance, making the concept unsuitable for extended unattended operation.
For those applications that require independent operation but only moderate to low amounts of power, a solid state device is preferred. Several approaches have been utilized in the past. The most successful to date has been the implementation of radiothermal generators, which use self-absorption in a block of radioactive material to generate heat. The thermal energy is then converted using thermovoltaic devices (e.g. thermocouples and thermoelectric generators). This technology is highly reliable and has been employed, for example, in space probes and in remote monitoring stations in the former USSR. However, the energy conversion is not very efficient, with maximum efficiency values of less than 10% and typical values of only a few percent. Furthermore, this technology can be scaled up, but does not scale down well to very small power applications because of increased heat loss as the ratio of surface area to volume increases.
A competing technology is the conversion of the radiation energy to electricity in a semiconductor junction, analogous to a solar cell. This can be done directly, by absorbing radiation in a semiconductor junction, or indirectly, by first converting the radiation to lower energy photons, namely ultraviolet or visible light. In principle, this can be done for any radioactive emission. However, beta emitters are typically preferred because of the short stopping distance for electrons compared with photons and the lower radiation damage potential compared with neutrons and alpha particles. This is particularly true for very small devices which may not be large enough to completely stop gamma rays. A solid state device that converts radiation to electrical power is called a nuclear battery. In the case of a device which utilizes pure beta radiation, the device would be called a beta-battery.
Several configurations have been demonstrated since beta batteries were first proposed in the 1950's. The most extensively used approach has been the direct approach of placing metal foils of beta particle emitters (such as nickel-63 and promethium-147) adjacent to the surface of a semiconductor diode so that the beta particles strike it, or exposing a semiconductor junction to a gas or liquid containing an emitter such as tritium. In a process analogous to a solar cell, the high energy beta particles generate a large number of electron hole pairs, which in turn generates a current through the diode.